This invention is directed generally to improvements in television receivers, and particularly to an improved network for altering the IF (intermediate frequency) response of a television receiver under weak signal conditions.
Television receivers include an IF amplifier which is generally provided with a bandpass characteristic in which the picture carrier (45.75 MH.sub.z) is positioned on one slope of the bandpass characteristic and about 6 db (decibels) below the peak response. The chroma subcarrier is typically positioned on the opposite slope of the bandpass characteristic and also about 6 db below the peak response.
Under normal signal conditions, the frequency response described above provides satisfactory results. However, under weak signal conditions, it has been found to be advantageous to modify the IF amplifier's frequency response such that the picture carrier is positioned at or near the peak response of the IF amplifier. The modified frequency response reduces picture detail, but the picture is generally "noisy" anyway under weak signal conditions so that picture detail would not readily be seen. The resultant picture is, nevertheless, subjectively more pleasing than one produced from an unmodified IF frequency response.
In the past, the IF frequency response has been modified or "walked" by including an additional tuned circuit downstream of the IF amplifier. The latter circuit is tuned to the picture carrier frequency and is designed to have a low Q (wide bandpass) under normal signal conditions and a high Q (narrow bandpass) under weak signal conditions. Hence, under normal signal conditions, the over-all frequency response of the IF amplifier is not changed substantially because of the wide bandpass of the additional tuned circuit. However, when signal strength decreases, the higher Q of the additional tuned circuit causes the picture carrier to be positioned at the peak of the over-all frequency response. The bandwidth of the over-all frequency response is also reduced by the effect of the high Q associated with the additional tuned circuit. A "walking" circuit of this type is disclosed in U.S. Pat. No. 3,872,387, assigned to the assignee of the present invention.
Although the walking circuit disclosed in the above-mentioned patent provides improved television performance, it has been found that even better performance results when the over-all bandpass is not narrowed when the picture carrier is elevated to the peak of the frequency response characteristic. In addition, the walking circuitry employed in the above-mentioned patent requires an extra transistor and is not readily manufacturable in integrated circuit form.
Other attempts have been made to "walk" the IF frequency response by a circuit which relies on changing the input impedance of a transistor to change the frequency response of a tuned circuit. The problem with the latter arrangement is that the input impedance of transistors of even the same type varies from unit to unit. Hence, inconsistent frequency response characteristics result.
Thus, prior IF "walking" techniques suffer from inconsistent results, an IF bandpass which is narrower than desired, or the inability to be easily integrated or manufactured inexpensively.